End-face reflected LiNbO3 waveguide based stationary miniature Fourier transform spectrometer with two-fold enhanced spectral resolution

Miniature Fourier transform spectrometer (FTS) has attracted considerable interest because of its important application in spaceborne spectroscopy and as a portable analytical tool for rapid on-site chemical/biochemical detection. In a previous paper, a stationary miniature FTS constructed with an electro-optic (EO) modulator of a LiNbO3 (LN) waveguide Mach-Zehnder interferometer (MZI) containing push-pull electrodes was demonstrated. This stationary miniature FTS is operated in the near-infrared region with either nonlinear or linear scanning of the modulating voltage. The simple and mirrorless structure renders the device compact, vibration resistant, and cost-effective. However, the spectral resolution of the proposed prototype FTS was not satisfactory due to the limited optical pathlength difference (OPD), thereby limiting the device application. To improve its spectral resolution, the factors affecting the spectral resolution of the LN waveguide-based FTS is investigated in this paper. Findings show that the spectral resolution is inversely proportional to the maximum OPD, which is proportional to the length of the EO modulating region. A simple method for two-fold enhancement of the spectral resolution of the FTS is proposed based on the end-face reflection in LN waveguide interferometer. With the end-face reflection geometry the guided mode can propagate back and forth in the LN waveguide, making the mode passing through the EO modulating region twice and consequently leading to two times enhancement of the OPD. Therefore, the end-face reflection geometry enables to double the maximum OPD of the interferometer without increasing the device size and thus to offer the device a two-fold enhanced spectral resolution according to the equation for FTS resolution. Two prototypes of FTS with and without the end-face reflection structure are prepared using the same commercial LN waveguide EO modulator. The spectral resolutions in terms of the full-width at half maximum (FWHM) at different wavelengths for the two prototypes of FTS are measured using a series of distributed feedback lasers. The FWHM measured at a specific wavelength with the end-face reflection structure is half as large as that obtained without the end-face reflection structure. Experimental results are in excellent agreement with the theoretical data, demonstrating the applicability of the end-face reflection method to the spectral resolution enhancement.